Impotence in the human male may be defined as the inability of the patient to achieve and/or sustain sufficient erection of the penis to permit sexual intercourse to be completed. Impotence is a surprisingly prevalent complaint among the adult male population and it is known that several different physiological and psychological factors may cause or influence impotence. In order to be able to treat the impotent patient it is of primary importance for the physician to determine whether the symptoms are physiological or psychological in origin. It is known that the normal male has three or four spontaneous erections of varying degrees during the course of a normal night's sleep so that if it can be determined whether or not the impotent patient is physically capable of an erection under rest conditions, the physician will have made a considerable advance in his diagnosis. Clearly as the patient is asleep, he cannot monitor his own bodily functions and heretofore the only way to make the required determination has been by measurement of penile expansion with mercury-in-rubber gauges connected to a polygraph. This equipment is expensive and the procedure cumbersome. Experience has shown that a minimum of three nights testing is required for reliable results and this is, therefore, a time consuming and expensive form of testing.